In personal and commercial watercraft, such as engine-driven fishing boats and speedboats, propulsion systems are often used in which a drive engine of the watercraft is arranged inside the watercraft and is actively connected, via a drive unit in the area of the watercraft's hull, to one or more propellers on the outside of the watercraft's hull. The drive unit typically passes through an opening in the hull or rear transom of the watercraft's hull, and usually includes an upper housing section or gear and clutch housing disposed inside the hull and having a drive shaft connected to an output shaft of the engine, and a lower housing section or underwater housing disposed outside the hull and having one or more output shafts connected to one or more propellers that rotate to drive the watercraft through the water. To avoid having the watercraft take on water through the opening in the hull, the drive unit must be secured within the opening with an arrangement that provides a sufficiently water-tight seal to prevent leakage when the watercraft is disposed and operated in the water.
One example of a known sealing arrangement is provided in U.S. Pat. No. 7,182,657, issued Feb. 27, 2007 to Mansson, which discloses a mounting arrangement wherein a boat hull is formed with a vertical well having an internal flange. The boat hull is used with a drive unit having a vertical drive shaft with an underwater housing connected to an upper gear housing, with the upper gear housing having a horizontal drive shaft for connection to an engine. A mounting plate of a drive unit and a screw-down plate are fastened to opposite sides of the internal flange, with compressible rings between surfaces of the mounting plate and the flange and between surfaces of the screw-down plate and the flange. The rings dampen vibrations from the drive to the hull. The propeller forces are transmitted via the mounting plate and the screw-down plate to the flange and the well and, thus, to the boat hull. The sealing arrangement of the Mansson patent may be effective in mounting the drive unit with the boat hull opening, but opportunities still exist for further improvements to such sealing arrangements.